Digital optical disks are capable of storing substantially greater amounts of information than conventional commercially available magnetic disk memories of comparable size. However, the presently commercially available digital optical disks are of the write-once type. These utilize a permanently deformable recording medium such as, for example, an ablatable material, wherein a high intensity laser write beam opens small holes in a threshold modifiable film coated upon a suitable substrate. Erasure is not possible.
Erasable digital magnetic disks, of the known commercially available variety, utilize magnetic medium wherein the film magnetization exhibits in-plane anisotropy. In view of the larger amount of real estate needed for each bit of information, it is clear that the stored information capacity would not be as large as that achievable with a system and medium capable of recording by means of perpendicular anisotropy.
The present invention relates to a system for writing, erasing and reading information upon a magneto-optic recording medium exhibiting perpendicular anisotropy. This means that when the portion of the medium in the information domain is heated to a sufficiently high temperature (as by exposure to a laser spot) in the presence of a magnetic field in the direction normal to the medium, the heated spots are caused to be oriented in the direction of lines of force in the magnetic field. Magnetization may be either in the UP direction or the DOWN direction, in accordance with the direction of the field lines passing therethrough. When this process is practiced with a gas laser, diffraction gratings have been required to create plural usable side-by-side spots, because of the large size of their optical output. Consequently, this has necessitated complex optics. With the advent of diode lasers of sufficient power output, it has been possible to direct usable closely spaced spots on the magneto-optic media through a single optical path. Such known systems have required precise alignment of the spatial relationship of the lasers, one to the other, in order to focus the plural spots through the same optical path.
It is now possible to build any number of diode lasers side-by-side on a chip. Thus, the lasers will be properly aligned as manufactured and the single optical path may be simplified. Most of the advantages of such a two laser system are achieved with no cost impact on the optics and a significant reduction in alignment problems. In U.S. Pat. No. 4,517,667, granted May 14, 1985, entitled "Direct Read After Write Optical Disk System", assigned to the same assignee as the instant application, Sprague describes a write-once recording and reading system, incorporating a single optical path, for use with a dual laser combined in a single package.